Differential dilation stent and method of use

ABSTRACT

Embodiments herein provide differential dilation stents for use in percutaneous interventions, such as transluminal bypass procedures. In some embodiments, the stents may be used in the process of creating an arteriovenous (AV) fistula during a percutaneous bypass procedure, and such stents may have two or more specialized regions that are configured to adopt a predetermined diameter, shape, and/or tensile strength upon insertion in order to suit the needs of the vessel or procedure. The disclosed stents may be used for creating and/or maintaining an arterial/venous fistula for bypass of an occlusion in a cardiac artery using a cardiac vein, or the femoral artery, for example using the tibial or popliteal vein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/422,594 (Attorney Docket No. 38338-709.201) filed Mar. 16, 2012,which claims the benefit of provisional application 61/453,876 (AttorneyDocket No. 38338-709.101), filed on Mar. 17, 2011, the full disclosuresof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments herein relate to methods and devices for connecting adjacentblood vessels, e.g., an artery and an adjacent vein, to adapt the veinfor arterial blood flow, and more particularly, embodiments relate todevices such as differential dilation stents for creating andmaintaining a pathway between an artery and a vein to form a fistulaconnecting the blood vessels.

The superficial femoral arteries and the popliteal arteries are legarteries that provide blood flow through the legs and to the feet,particularly to the skin and areas just below the skin. Patientssuffering from partial or complete occlusions in such arteries typicallyexperience claudication (e.g., leg pain or limping while walking) anddifficulty in healing wounds on the legs due to ischemia, although thedeep femoral artery may provide enough circulation that at least thepain is reduced by resting. However, standard open bypass often isimpossible on such patients, particularly those with diabetes-narrowedarteries, because of the substandard ability to heal the necessaryincisions. Neither performing angioplasty nor inserting stents is likelyto help where the vessels are too small or the occlusion extends all theway down to the foot. In severe cases, non-healing ulcers or restingpain may leave no alternative except amputation. Thus, peripheralvascular disease presents a serious health risk.

2. Description of the Background Art

Commonly assigned and invented U.S. Pat. No. 7,300,459 describes a stentwith a covering that can be differentially dilated.

SUMMARY OF THE INVENTION

The present invention provides stents suitable for implantation in humanblood vessels, including both arteries and veins. The stents aretypically in the form of a stent-graft including an inner scaffoldmember, typically formed from a metal structure such as a cylindricalmesh body having an inside surface and an outside surface. Thecylindrical mesh body or other scaffold component will be covered,preferably over both the inside and outside surfaces, with a flexiblecoating which may comprise a polytetrafluoroethylene (PTFE material),such as electrospun PTFE, an ultra high molecular weight polyethylene(UHMWPE material), or the like. In both cases, the coating material willtypically be in the form of a thin membrane or sheet, and small holes,such as laser-drilled holes, may be formed through the material in orderto promote cellular in-growth. Such holes will typically have a diameterin the range from 5 microns to 30 microns. The inner stent or scaffoldcomponent may be formed from either balloon-expandable materials andself-expandable materials. Preferably, however, the stent or scaffoldwill be in the form of a super elastic metal, such as nickel titaniumalloy.

In particular embodiments, the stent or scaffold body will be formed ortreated to include at least two regions having different strengths,particularly having greater or lesser hoop strengths. It has been foundthat providing the regions near each end of the stent with a greaterhoop strength than the region(s) in the middle is useful when stents aredeployed between blood vessels, where the end pass through openingsbetween the blood vessels that can provide relatively high compressiveforces which can collapse the stents or stent-grafts. Usually a regionbetween the reinforced which passes through a vein will have a diameterless than that of the vein to allow continuous venous blood flow afterthe bypass is in place. Thus, exemplary stents and stent-grafts can havea first region with a high tensile strength or hoop strength and adiameter selected to extend across the arterial lumen proximal to theocclusion, a second region with a lesser tensile or hoop strength and adiameter usually less than that of the vein adjustment to the arterialocclusion, and finally at least a third region having a greater tensileor hoop strength than the second region and a diameter selected toextend fully across the arterial lumen distal to the occlusion. Theseregions are preferably arranged with higher strength regions at each endand the lower strength region between said ends.

Exemplary dimensions for the stents and stent-grafts of the presentinvention include a length from a proximal end to a distal end of thestent or stent-graft from 40 cm to 150 cm. With such lengths, the firstregion will typically have a diameter from about 4 mm to 9 mm, and alength from 10 cm to 20 cm. The second region will have a diameter fromabout 6 mm to 11 mm and a length in the range from about 25 cm to 45 cm.The third region will have a diameter in the range from 2 mm to 6 mm anda length in the range from 15 cm to 25 cm. These dimensions areparticularly suitable for placing the stent from a femoral artery, intoan adjacent femoral vein and/or poplitel vein, back into the femoralartery, typically to bypass an occlusion.

In other embodiments, the distance from the proximal end to the distalend may be in the range from 35 cm to 45 cm, where the first region hasa diameter from 6 mm to 8 mm and the third region has a diameter from 4mm to 6 mm. In yet another embodiment, the distance from the proximalend to the distal end is about 40 cm and the first region has a diameterof 7 mm and the third region has a diameter of about 5 mm. In yetanother embodiment, the distance from the proximal to distal end of thestent is in the range from 55 cm to 65 cm in the first region, has adiameter in the range from 5 mm to 7 mm and the third region has adiameter from 2 mm to 4 mm. In yet another embodiment, the distance fromthe proximal end to the distal end of the stent is about 60 cm. Thefirst region has a diameter of about 6 mm and the third region has adiameter of about 3 mm.

In other specific embodiments, the mesh body or other stent scaffold maybe pre-formed or biased to have at least one curve, optionally includinga single S-shaped curve, and further optionally including a pair ofS-shaped curves which are arranged with the first curve located near aproximal end of the stent and a second curve located near a distal endof the stent.

In particular designs of the stents of the present invention, the meshbody or scaffold will comprise a plurality of serpentine rings arrangedin a longitudinally successive pattern. Each serpentine ring willinclude elongate struts joined by arcuate joints, and the adjacent ringswill themselves be arranged in a generally continuous helical pattern(resembling a helical ribbon wrapped over a cylinder) with a helical gapbetween the adjacent ends of each ring.

In a first specific embodiment, at least one of the two terminalserpentine rings will have a tail wire, which is attached along itslength to an adjacent strut. Such a structure can minimize loading andmovement of the tail wire relative to the cover material of the stent.In a specific embodiment, the adjacent strut which is attached to thetail wire can itself extend in to a second and even a third ring movinginwardly from the terminal end with such an elongated stent beingfurther attached to the second and optionally third adjacent serpentinering.

In other preferred embodiments of the scaffold or cylindrical mesh body,a longitudinal gap will be maintained between at least most of theopposed ends of adjacent arcuate joints. In particular, if a helicalline is drawn between circumferentially adjacent arcuate joints on afirst serpentine ring, and a second helical line drawn on the opposedends of the immediately adjacent serpentine ring, then these first andsecond helical lines will maintain a minimum separation, typically inthe range from 0.01 inch (0.25 mm) to 0.05 inches (1.25 mm), frequentlybeing about 0.025 inches. Such caps allow the stent to accommodate atight bending radius, typically of at least 0.6 inches (15 mm). While alarger gap would allow for even tighter bending, a gap in the range setforth above will provide both a desired radial strength and a desiredbendability.

Additionally, the adjacent arcuate joints will usually becircumferentially offset so that the joint on one serpentine ring willbe aligned with the gap between joints on the adjacent serpentine ring.Such “out-of-phase” joints reduce wear and interference which mightoccur if the joints were aligned so that they would interfere (collide)with each other when the stent is bent.

In still other aspects of the present invention, the flexible coatingwill comprise at least one sheet or membrane on each side of thescaffold or cylindrical mesh body structure in order to fullyencapsulate the scaffold structure. Additionally, it will often bepreferred to cut-out or “scallop” the end of the flexible coating tomatch the undulations of the terminal serpentine ring. Further, it willfrequently be desirable to coat the flexible coating with a drug, suchas heparin, on the outer surface, the inner surface, or both.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be readily understood by the following detaileddescription in conjunction with the accompanying drawings. Embodimentsare illustrated by way of example and not by way of limitation in thefigures of the accompanying drawings.

FIG. 1 is a partial cross-sectional view showing an obstructed artery,including the obstruction and the area adjacent both ends of theobstruction, and a vein alongside the artery, in accordance with variousembodiments;

FIG. 2 illustrates a pair of temporary prop (non-covered) stents placedproximally and distally in an anastomosis created between the obstructedartery and adjacent vein shown in FIG. 1, in accordance with variousembodiments;

FIG. 3 illustrates a differential dilation self-expanding stent placedinside the temporary stents shown in FIG. 2 and bridging a bypassbetween the artery and vein graft, in accordance with variousembodiments; and

FIG. 4 illustrates a modular differential dilation self-expanding stentplaced inside the temporary stents shown in FIG. 2 and bridging a bypassbetween the artery and vein graft, in accordance with variousembodiments.

FIG. 5 illustrates a detail of an exemplary stent scaffold constructionin accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which are shownby way of illustration embodiments that may be practiced. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope. Therefore,the following detailed description is not to be taken in a limitingsense, and the scope of embodiments is defined by the appended claimsand their equivalents.

Various operations may be described as multiple discrete operations inturn, in a manner that may be helpful in understanding embodiments;however, the order of description should not be construed to imply thatthese operations are order dependent.

The description may use perspective-based descriptions such as up/down,back/front, and top/bottom. Such descriptions are merely used tofacilitate the discussion and are not intended to restrict theapplication of disclosed embodiments.

The terms “coupled” and “connected,” along with their derivatives, maybe used. It should be understood that these terms are not intended assynonyms for each other. Rather, in particular embodiments, “connected”may be used to indicate that two or more elements are in direct physicalor electrical contact with each other. “Coupled” may mean that two ormore elements are in direct physical or electrical contact. However,“coupled” may also mean that two or more elements are not in directcontact with each other, but yet still cooperate or interact with eachother.

For the purposes of the description, a phrase in the form “A/B” or inthe form “A and/or B” means (A), (B), or (A and B). For the purposes ofthe description, a phrase in the form “at least one of A, B, and C”means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B and C).For the purposes of the description, a phrase in the form “(A)B” means(B) or (AB) that is, A is an optional element.

The description may use the terms “embodiment” or “embodiments,” whichmay each refer to one or more of the same or different embodiments.Furthermore, the terms “comprising,” “including,” “having,” and thelike, as used with respect to embodiments, are synonymous.

Embodiments herein provide differential dilation stents for use inpercutaneous interventions, such as peripheral transluminal bypassprocedures. In some embodiments, the stent may be used in the process ofcreating an arteriovenous (AV) fistula during a percutaneous bypassprocedure, and such stents may have two or more specialized regions thatare configured to adopt a predetermined diameter, shape, and/or tensilestrength upon insertion in order to suit the needs of the vessel orprocedure. An example of such a procedure is described herein for abypass in a generic artery, but one of skill in the art will recognizethat the approach may be adapted for use in the treatment for peripheralvascular disease in any peripheral artery, such as in an iliac artery,superficial femoral artery, common femoral artery, popliteal arter,posterior tibial artery, perineal artery, or anterior tibial artery.

FIG. 1 is a simplified diagram of a portion of the peripheralvasculature 10 illustrating a vein 12 and an artery 14 having anobstruction or restriction 16 disposed therein. In such vasculature 10,if such a restriction 16 develops in artery 14, the delivery ofoxygenated blood to the periphery (e.g., limb) distal to restriction 16may be significantly compromised. Accordingly, it may be desirable toestablish an alternative pathway for the oxygenated blood to flow to theperiphery, for example using percutaneous bypass.

With reference to FIG. 2, an alternative pathway 20 for blood flow maybe established by a pair of arterial/venous (AV) fistulas 18, which maybe in the form of a vascular graft or the like, and which may provide aroute for the bloodflow to bypass restriction 16. In variousembodiments, alternative pathway 20 may include a proximal end 22, adistal end 24 and a lumen 26 extending therethrough. In variousembodiments, alternative pathway 20 may pass through arterial exit hole28 and into venous entry hole 30, through a portion of vein 12, intovenous exit hole 32, and out arterial re-entry hole 34 and back intoartery 14. With this arrangement, blood flowing into artery 14 may flowinto the proximal end 22 of pathway 20, through lumen 26, and may exitthe distal end 24 of pathway 20, thus delivering oxygenated blood to theperiphery. Thus, in various embodiments, pathway 20 may provide analternative path for the oxygenated blood to flow around restriction 16.A detailed description of this technique is disclosed in InternationalU.S. Pat. No. 7,374,567, which is hereby incorporated by reference.

Although the general method of bypassing a restriction in an artery byproviding a conduit via an adjacent vein is known, the percutaneoustransluminal technique may be particularly difficult from a devicestandpoint due to the potentially long distance between the vascularaccess site and the treatment site, the relatively small lumen size ofthe vascular path, and the precise control required for the technique ina dynamic environment. As used herein, the term “extravascular opening”may include any one or a combination of the following: an arterial entryor re-entry, an arterial exit, a venous entry or re-entry, and/or avenous exit. In a similar manner, the term “pathway” as used herein mayinclude, without limitation, a pathway established external to anartery, internal to an artery, through an opening in an artery, and/orin the case of adjacent vessels, the pathway may be defined by theopenings in the vascular wall(s) and passage through the vessel.

With reference to FIG. 2, in various embodiments, a general peripheralpercutaneous transluminal bypass procedure may be described as follows.First, a suitable device may be navigated to the vasculature usingconventional methods. For example, access to the site of the occlusionmay be established by way of an artery, such as an ipsilateral orcontralateral femoral artery, for example, using an access sheath andguide catheter navigated using a guidewire. Once the desired device isnavigated to the treatment site proximal of restriction 16 in the artery14, the treating physician may determine the correct penetration site tocreate an extravascular opening. In various embodiments, a tissuepenetrating device then may create the extravascular opening in thevascular wall of artery 14, and may be further used to define a pathwayto vein 12. In various embodiments, the process of creating theextravascular opening and the pathway may be monitored to ensure that aproper position for the extravascular opening and pathway areestablished.

In various embodiments, once the extravascular opening in artery 14 isestablished and a pathway to the vein 12 is defined, an extravascularopening may then be established in vein 12. In some embodiments, thismay be accomplished utilizing the same tissue penetrating device. Thecreation of the extravascular opening in vein 12 may also be monitoredto ensure that a proper venous access opening has been established. Invarious embodiments, after creating an opening in artery 14 and apathway to an opening in vein 12, the openings and pathway may bemaintained by a suitable means such as a vascular graft or stent 36(e.g., a self-expanding or balloon-expandable stent). The stents andgrafts will usually be intended for permanent implantation. In someinstances, placeholder stents may be used before placement of apermanent stent or graft, where the placeholder may optionally beremoved. This may establish fluid communication between artery 14proximal of restriction 16 to vein 12, as illustrated in FIG. 2.

With reference to FIG. 2, in various embodiments, placeholder stent 36may be a smooth, fairly flexible stent with a mild amount of tensilestrength, such as a self-expanding, non-covered mesh (e.g. Nitinol™)stent. In some embodiments, placeholder stent 36 may be adapted to adoptan “S” curve from artery 14 into and across the anastomosis to vein 12.In particular embodiments, placeholder stent 36 may then adopt a second“S” curve distally from vein 12 and back into artery 14 after the returnopening is established in vein 12 and artery 14 as described below,whereas in other embodiments a second “S” shaped placeholder stent 36may be used for this purpose. In some embodiments, placeholder stent 36may be biased to adopt the “S” shape, for example by varying thethickness of the mesh wall. For example, in one embodiment, placeholderstent 36 may be biased toward a modified “S” shape by shaping the wallof the stent along the convex curvature to be a little thinner (orhaving a single layer or thinner mesh weave, for example) and along theconcave curvature a little thicker (or having a double layer or thickermesh weave, for example) to cause the bias. In some embodiments,placeholder stent 36 may be further dilated with a differential dilationstent as described in more detail below.

In various embodiments, the next steps involve establishing a fluid pathbetween vein 12 and artery 14 distal to restriction 16. This may beaccomplished, in various embodiments, by creating an extravascularopening in vein 12, defining a pathway to artery 14, creating anextravascular opening in artery 14 distal to restriction 16, monitoringthe progress of the creation of the extravascular openings and pathway,and providing a device, such as a placeholder stent 36, to maintain thefluid path once established.

Although this procedure is described herein in general terms, one ofskill in the art will appreciate that the procedure may be modified forformation of an arterial/venous bypass in any part of the body, forexample in an iliac bypass (e.g., a bypass spanning a blockage in aniliac artery), in a superficial femoral bypass (e.g., a bypass spanninga blockage in a superficial femoral artery), in a common femoral bypass(e.g., a bypass spanning a blockage in a common femoral artery), in apopliteal artery (e.g., a bypass spanning a blockage in a poplitealartery), in a posterior tibial bypass (e.g., a bypass spanning ablockage in a posterior tibial artery), in a perineal bypass (e.g., abypass spanning a blockage in a perineal artery), or in an anteriortibial bypass (e.g., a bypass spanning a blockage in an anterior tibialartery). Approaches to such arteries are known to those of skill in theart, and may include any convenient artery that has a diameter that issufficiently large to admit the catheters, stents, and guidewiresrequired for the procedure. In some embodiments, the blockage may beaccessed via an ipsilateral or contralateral femoral artery.

In various embodiments, the device provided to maintain the fluid pathonce established using the placeholder stent may be a differentialdilation stent as described herein. FIG. 3 illustrates an example ofsuch a differential dilation stent 40 in use spanning an arterialocclusion 16. As shown in FIG. 3, differential dilation stent 40 mayinclude a self-expanding mesh structure 42 having a generallycylindrical shape that is configured to contact an interior lining orwall of artery 14 and/or vein 12 when expanded. In various embodiments,differential dilation stent 40 may include a film layer on both theinside and outside surfaces, which, in some examples, may be made of aflexible material such as polytetrafluoroethylene (PTFE) or ultra highmolecular weight polyethylene (UHMWPE), for example. In variousembodiments, differential dilation stent 40 may have a nominal shapethat is generally cylindrical, including first (proximal) and second(distal) opposed ends 44, 46 communicating with an interconnectingcentral lumen 48. The inner and outer film layers may be substantiallythe same length and may be connected or sealed together, particularlyadjacent ends 44, 46. In some embodiments, mesh layer 42 may be shorterin length than the inner and outer film layers, for example by about 0.5mm, although is some embodiments mesh layer 42 may be substantially thesame length as the inner and outer film layers.

In various embodiments, when extended, differential dilation stent 40may have a length from first (proximal) end 44 to second (distal) end 46of from about 20 cm to about 150 cm, or even longer, for instancedepending on the anatomy of the patient, and may have a diameter ofabout 2 mm-12 mm in various regions or portions, depending on theanatomy of the patient, the site of the occlusion, and the particularartery and/or vein. In some embodiments, differential dilation stent 40also may include two or more different regions, such as a proximalregion 50, a central region 52, and a distal region 54. In variousembodiments, the diameter, length, and tensile strength (e.g.,flexibility, firmness, and/or expansion force) of each region may beindividually configured to suit a particular medical use. For example,differential dilation stent 40 may be configured to have more tensilestrength and a greater diameter proximally near the site of theanastomosis (e.g., in proximal region 50) and less tensile strength anda larger diameter centrally near the vein graft (e.g., in central region52) where the diameter of the vessel is larger and the vessel walls aremore compliant. In some embodiments, distal region 54, which re-entersand passes through artery 14, may have a tensile strength similar tothat of proximal region 50, but may have a smaller diameter than centralregion 52. In some embodiments, having a more compliant, softer, and/orflexible central region 52 may reduce the likelihood of damage to andreduces stress on vein 12, which may have more compliant walls thanartery 14. Conversely, in some embodiments, having a less compliant,more rigid proximal or distal end may help with directing differentialdilation stent 40 through the vessel walls and may retain theanastomosis in an open configuration.

Conventional self-expanding stents normally expand to fit the dimensionsof the vessel they are seated in, but in various embodiments, the stentsof the present disclosure may not necessarily expand all of the way tospan the inner walls of the vessel. For example, in one embodiment,differential dilation stent 40 may have a proximal region 50 that isabout 4-10 mm in diameter and about 10-30 cm in length, while distalregion 54 may be about 2-8 mm in diameter and may be about 10-20 cmlong. In this embodiment, central region 52 may have a diameter of about5-12 mm, and may have a length of about 20-100 cm in length.

In one specific, non-limiting embodiment, for example, for afemoral-popliteal bypass, differential dilation stent 40 may have anoverall length of about 80 cm, for example, a proximal region 50 with adiameter of about 6-8 mm, for example, about 7 mm, a central region 52with a diameter of about 5-12 mm, for example, about 8 mm, and a distalregion 54 with a diameter of about 4-6 mm, for example, about 5 mm.

In another specific, non-limiting embodiment, for example, for afemoral-tibial bypass, differential dilation stent 40 may have anoverall length of about 55-65 cm, for example, about 60 cm, a proximalregion 50 with a diameter of about 5-7 mm, for example, about 6 mm, acentral region 52 with a diameter of about 5-12 mm, for example, about 8mm, and a distal region 54 with a diameter of about 2-4 mm, for example,about 3 mm.

In many embodiments, it will be desirable that the portion of the stentor graft in the vein have a diameter which is less than that of the veinso that a portion of the venous lumen remains available for normalvenous blood flow.

The tensile strength or expansion force of differential dilation stent40 in different regions (e.g., proximal, central, and distal) may becontrolled in several ways, in accordance with various embodiments. Forexample, the thickness of the nitinol wire used to weave the wire meshlayer of differential dilation stent 40 may be varied, with a thinnerwire used in more compliant regions (e.g., central region 52), and athicker wire used in less compliant regions (e.g., proximal and distalregions 50, 54) in some embodiments. Alternately or additionally, thewire mesh may be formed from a more expandable pattern or a double weavein more compliant regions (e.g., central region 52), and a lessexpandable pattern in more compliant regions (e.g., proximal and distalregions 50, 54) in accordance with various embodiments. Further, in someembodiments, the wire mesh layer may be formed using two or moredifferent materials with different inherent resistances to expansion,for example incorporating a blend of nitinol and steel in the lesscompliant regions (e.g., proximal and distal regions 50, 54).

In some embodiments, differential dilation stent 40 may be adapted toautomatically adopt an “S” curve (or other curved shape) from the arteryinto and across the anastomosis to the vein. In particular embodiments,differential dilation stent 40 may be adapted to then adopt a second “S”curve distally from the vein and back into the artery after the returnopening is established in the vein 12 and artery 14. In someembodiments, differential dilation stent 40 may be biased to adopt the“S” shape (or other curve), for example by varying the thickness of themesh wall. For example, in one embodiment, the placeholder stent may bebiased toward a modified “S” shape by shaping the wall of the stentalong the convex curvature to be a little thinner and along the concavecurvature a little thicker to cause the bias.

In other embodiments, as illustrated in FIG. 4, a set of modular stents(e.g., proximal stent 60, central stent 62, and distal stent 64) may beused in place of the single differential dilation stent 40 describedabove. In various embodiments, the dimensions and/or tensile strength ofproximal stent 60, central stent 62, and distal stent 64 may generallycorrespond to the tensile strength and/or dimensions of proximal 50,central 52, and distal 54 regions of differential dilation stent 40 asdescribed above. In various embodiments, two firmer modular stents 60,64 may be placed proximally and distally of restriction 16,respectively, and connecting artery 14 to vein 12 on both sides of therestriction 16, and then vein 12 may receive a more compliant stent 62,for example made of PTFE or Dacron, to reduce or eliminate stretching ofvein 12.

In various embodiments, modular stents 60, 62, and 64 may be placed withor without placeholder stents 36. In various embodiments, proximal stent60, which may have a high tensile strength (e.g., it may be highlyreinforced to maintain the proximal anastomosis) it may have an expandeddiameter of about 5-10 mm, or about 7 mm in some examples. In variousembodiments, because central stent 62 may be positioned within the morecompliant vein, it may be more compliant (e.g., have a lower tensilestrength) like central portion 52 of differential dilation stent 40. Invarious embodiments, central stent 62 may have an expanded diameter offrom about 5 mm to about 12 mm. The length of central stent 62 may befrom about 20 cm to as much as 80 cm or even 100 cm in some embodiments.In various embodiments, distal stent 64 may be reinforced (e.g., have ahigh tensile strength) since it may pass through and maintain the distalanastamosis, and may have an expanded diameter of from about 3 mm toabout 8 mm, or about 5 mm in certain examples. Distal stent 64 also mayhave a length of about 10 cm to 20 cm in length.

In further embodiments, differential dilation stent 40 (or modularstents 60, 62, 64) may be provided with one or more radiopaque portions,for example, adjacent first and second ends 44, 46. Thus, in someembodiments, after differential dilation stent 40 (or modular stents 60,62, 64) has been installed and expanded in a human blood vessel,differential dilation stent 40 (or modular stents 60, 62, 64) may beobserved under an X-ray and the relative locations of ends 44, 46 may bedetermined.

Referring now to FIG. 5, a stent 70 comprises a stent body 72 in theform of a plurality of serpentine rings each including axial struts 74joined by arcuate joints 76. Although the rings will be referred toindividually, they will usually comprise a continuous helical structurewith only one distal end and one proximal end, particularly when theyare formed as a bent wire. When formed by laser cutting, the rings couldbe independent but will usually be a continuous helical structure. Theserpentine rings are disposed between at least one inner membrane or alayer 75 of a graft material and at least one outer layer (not shown),where the inner and outer layers are joined together to encapsulate thescaffold structure 72. At at least one end of the stent 70, the layersof graft material 75 will usually be “scalloped” so that the graftterminates in an undulating pattern which matches the pattern of theterminal serpentine ring.

In order to enhance the radial strength (hoop strength) of the scaffoldstructure 72 at at least one end (and preferably at both ends), the tailstrut 80 may be welded or otherwise attached to the adjacent strut 82.To further enhance the strength of this end, the adjacent strut 82 mayhave a length sufficient to span the terminal ring as well as theadjacent ring which is one position inward from the terminal layer. Thisstrut 82 will preferably be joined to the adjacent strut 84, and strut84 itself may be lengthened to transition into the remainder of theserpentine rings.

Except for the struts 82 and 84 which span adjacent serpentine ringsstructures, there will usually be a gap G maintained between the arcuateends 74 of the adjacent serpentine rings. This gap will be maintainedwithin the ranges discussed above in order to enhance bendability of thestent structure while maintaining sufficient radial or hoop strength.

Although certain embodiments have been illustrated and described herein,it will be appreciated by those of ordinary skill in the art that a widevariety of alternate and/or equivalent embodiments or implementationscalculated to achieve the same purposes may be substituted for theembodiments shown and described without departing from the scope. Thosewith skill in the art will readily appreciate that embodiments may beimplemented in a very wide variety of ways. This application is intendedto cover any adaptations or variations of the embodiments discussedherein. Therefore, it is manifestly intended that embodiments be limitedonly by the claims and the equivalents thereof.

What is claimed is:
 1. A stent configured for insertion in a human blood vessel, the stent comprising: a bent wire formed into a cylindrical body comprising a plurality of serpentine rings including a terminal ring at one end of the body and an adjacent ring inward from the terminal ring, wherein each serpentine ring comprises a plurality of axial struts, wherein circumferentially adjacent axial struts are joined by arcuate joints and wherein each serpentine ring has an inside surface, an outside surface, a proximal end, and a distal end, and wherein at least some of the axially adjacent serpentine rings other than the terminal ring and inward ring maintain a longitudinal gap between ends of adjacent arcuate joints.
 2. The stent of claim 1, wherein the terminal ring ends in a tail strut and at least one strut in the terminal ring other than other than the tail strut is longer than all other struts in the terminal ring and has a length sufficient to span the terminal ring and the adjacent ring inward from the terminal ring to create an overlap which enhances the strength of the ends of the body.
 3. The stent of claim 2, further comprising a flexible coating including at least one sheet or membrane covering the inside and outside surfaces of at least some of the serpentine rings in order to encapsulate the body.
 4. The stent of claim 3, wherein the flexible coating fully encapsulates the cylindrical body.
 5. The stent of claim 4, wherein an end of the flexible coating is scalloped so that the coating terminates in an undulating pattern which matches the pattern of the terminal ring.
 6. The stent of claim 3, wherein the flexible coating comprises polyethylene terephthalate (PTFE).
 7. The stent of claim 1, wherein the bent wire comprises nickel titanium alloy.
 8. The stent of claim 1, wherein the bent wire is formed into a continuous helical structure with only one distal end and one proximal end.
 9. The stent of claim 1, wherein the cylindrical body has a length in the range from about 40 cm to about 150 cm.
 10. The stent of claim 1, wherein the cylindrical body is self-expanding and biased toward an open position.
 11. The stent of claim 1, wherein the cylindrical body is biased toward a curved position.
 12. The stent of claim 11, wherein the curved position comprises a substantially S-shaped curve.
 13. The stent of claim 11, wherein the curved position comprises two substantially S-shaped curves.
 14. The stent of claim 13, wherein a first substantially S-shaped curve is located near the proximal end, and wherein a second substantially S-shaped curve is located near the distal end.
 15. The stent of claim 1, wherein the tail strut in the terminal ring is attached along its length to the at least one adjacent strut.
 16. The stent of claim 15, wherein the at least one adjacent strut in the terminal ring extends axially to the adjacent ring and is attached to a strut in the adjacent ring which spans a third ring.
 17. The stent of claim 3, wherein small holes are formed through the flexible coating. 